Internally the frSky uses the Bosch BMP085 BARO sensor
FrSky FVAS-01 Telemetry Variometer Sensor (for use with FrSky Telemetry Sensor Hub)
Measurement Range of Altitude: 0~9000m
Installation: Use double-sided tape to install the variometer sensor on any appropriate surface of the airframe in neutral static pressure zone.
*Note: Wind/Airpressure will affect the altitude reading.